1. Field of the Invention
The present invention relates to a residual charge erasing method for a solid state radiation detector. The solid state radiation detector includes an electrostatic recording section having a photoconductive layer that shows conductivity when exposed to recording light, and is constructed to receive recording light representing image information to record the image information therein and to output image signals representing the recorded image information. More specifically, the present invention relates to a residual charge erasing method for erasing unwanted charges remaining in the photoconductive layer of such solid state radiation detectors. The present invention also relates to a radiation image recording/readout apparatus.
2. Description of the Related Art
Today, in X-ray (radiation) imaging for medical diagnosis and the like, various types of X-ray image recording/readout systems are proposed and put into practical use. Such system uses a solid state radiation detector(detector that employs semiconductors in the main section) as the X-ray image information recording means, and X-rays transmitted through a subject are detected by the solid state detector to obtain image signals representing the X-ray image of the subject.
Various types of detectors are also proposed as the solid state radiation detectors for use in such systems. For example, from the aspect of charge generating process in which X-rays are converted to charges, photoelectric conversion type solid state detectors, direct conversion type solid state detectors, and the like are proposed. In the photoelectric conversion type solid state detector, fluorescent light emitted from a phosphor when X-rays are irradiated thereon is detected by a photoconductive layer to obtain signal charges, which are temporarily stored in the storage section, then the stored charges are outputted after converted to image signals (electrical signals). In the direct conversion type solid state detector, signal charges generated in the photoconductive layer when X-rays are irradiated thereon are collected by charge collection electrodes and temporarily stored in the storage section, thereafter the stored charges are outputted after converted to electrical signals. In the solid state detector of this type, the photoconductive layer and the charge collection electrodes constitute the main section.
From the aspect of charge readout process in which the stored charges are read out to outside, optical readout type detectors in which the stored charges are read out by irradiating readout light (readout electromagnetic wave) on the detector, TFT readout type detectors as described, for example, in U.S. Pat. No. 6,828,539 in which the charges are read out by scan driving TFTs (thin film transistors) connected to the storage section, and the like are proposed.
A modified direct conversion type solid state detector is also proposed by the inventor of the present invention in U.S. Pat. No. 6,268,614. The modified direct conversion type solid state detector proposed by the inventor is a direct conversion/optical readout type solid state detector. The detector includes the following layers arranged in the order listed below: a recording photoconductive layer that shows conductivity when exposed to recording light (X-rays, fluorescent light generated by the irradiation of X-rays, and the like); a charge transport layer that acts as substantially an insulator against charges having the same polarity as latent image charges and as substantially a conductor for transport charges having the opposite polarity to that of the latent image charges; and a readout photoconductive layer that shows conductivity when exposed to readout light. Here, signal charges (latent image charges) that represent image information are stored in the interface between the recording photoconductive layer and charge transport layer. An electrode layer (first conductive layer or second conductive layer) is provided on each side of the three-layer composite. In this type of solid state detector, the recording photoconductive layer, charge transport layer, and readout photoconductive layer constitute the main section of the detector.
Basically, after the latent image charges are read out, each of the solid state detectors described above is supposed to have no latent image charges remaining therein, and to be directly usable for the next recording. There may be cases, however, that the latent image charges are not read out completely and some of them remain in the detector. Further, when recording a latent image in the solid state detector, a high voltage is applied to the detector before the irradiation of recording light, which causes dark currents to flow and unwanted charges (dark current charges) are also accumulated in the detector. Still further, it is known that unwanted charges are accumulated in the detector due to various other reasons prior to the irradiation of recording light.
The residual charges including charges not read out from the detector and unwanted charges accumulated in the detector prior to the irradiation of recording light are added to the latent image charges stored in the detector through the irradiation of recording light. Consequently, when the electrostatic latent image charges are readout from the solid state detector, the output signal includes the signal components of the residual charges, as well as those based on the latent image charges representing image information, causing the problems of residual image or degraded signal-to-noise ratio.
For this reason, the inventor of the present invention has proposed a residual image erasing method in U.S. Pat. No. 6,268,614, in which erasing light is irradiated on the solid state detector prior to the irradiation of recording light while the voltage is being applied to the detector to erase the residual charges accumulated in the storage section in advance.
It is difficult, however, to completely erase the residual charges by the conventional method, and more effective residual charge erasing methods are needed.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide a more effective residual charge erasing method for a solid state radiation detector which includes an electrostatic recording section having a photoconductive layer that shows conductivity when exposed to recording light, and is constructed to receive recording light representing image information to record the image information therein, and to output image signals representing the recorded image information. It is a further object of the present invention to provide a radiation image recording/readout apparatus.